Introduction
An amperage number is only useful when it turns into hardware: the right wire gauge, the right breaker, the right panel — and the connectors, boxes, and conduit that tie them together. Undersize any link and it becomes the weak point; oversize everything blindly and you waste money on copper. This guide maps current ranges to the materials that carry them, with the selection logic professionals use.
Start from the load: the Amperage Calculator converts your watts and volts into amps. Once you have that number (plus the 80% margin for continuous loads), this guide tells you what to buy.
Wire: Matching Gauge to Current
Wire ampacity — the current a conductor can carry continuously without exceeding its temperature rating — is the foundation. Start from the load current computed by the complete guide method; for copper conductors in typical residential conditions (per NEC Table 310.16, before derating):
| AWG (copper) | Baseline ampacity | Typical applications |
|---|---|---|
| 14 AWG | 15 A | Lighting, general receptacles |
| 12 AWG | 20 A | Kitchen/bath/laundry receptacles |
| 10 AWG | 30 A | Water heaters, dryers, small A/C |
| 8 AWG | 40–50 A | Ranges, subpanel feeders |
| 6 AWG | 55–65 A | Subpanels, large feeders |
| 4 AWG and larger | 70 A+ | Service entrances, large feeders |
Copper vs aluminum
Copper is the default for branch circuits: smaller gauge for the same amps, easier terminations, no special compounds. Aluminum is common for large feeders and service entrances (it’s significantly cheaper per amp at large gauges) but requires CO/ALR-rated or aluminum-rated terminations and anti-oxidant compound, plus an installer experienced with it. Never mix metals at a termination not rated for both.
Derating: when baseline ampacity doesn’t apply
Baseline values assume moderate ambient temperature and no more than three current-carrying conductors in a raceway. Reality often differs:
- High ambient temperature. Attics can exceed 130°F (55°C) in summer; NEC correction factors reduce ampacity as temperature rises. A wire rated 20 A at 86°F (30°C) may be rated only ~17 A at 122°F (50°C).
- Conduit fill. More than three current-carrying conductors in one conduit triggers adjustment factors (e.g., 80% for 4–6 conductors). Long conduit runs with many circuits need upsized wire.
- Long runs. Voltage drop, not ampacity, often governs past ~100 feet (30 m) — check the Voltage Drop Calculator and upsize if drop exceeds ~3% on branch circuits.
Selection rule: compute the load current, apply the 80% rule for continuous loads, apply derating factors, then choose the smallest wire whose derated ampacity still exceeds the result. The code compliance guide covers the NEC references behind each step.
Breakers: Protecting the Wire
The breaker exists to protect the wire — its rating must not exceed the wire’s ampacity (NEC 240.4, with specific exceptions). Standard residential breaker sizes: 15, 20, 25, 30, 35, 40, 45, 50, 60 A.
| Load current | Wire (copper) | Breaker | Notes |
|---|---|---|---|
| ≤ 12 A continuous | 14 AWG | 15 A | 80% of 15 A = 12 A |
| 12–16 A continuous | 12 AWG | 20 A | The 1,800 W heater (15 A) lands here |
| 16–24 A continuous | 10 AWG | 30 A | 80% of 30 A = 24 A |
| 24–32 A continuous | 8 AWG | 40 A | Check terminal temperature ratings |
Breaker types that matter
Beyond the amp rating: GFCI breakers (or receptacles) are required in wet locations — kitchens, bathrooms, garages, outdoors. AFCI protection is required on most residential 15/20 A branch circuits under current NEC. HACR/time-delay ratings matter for motor loads with high inrush. A standard thermal-magnetic breaker is fine for resistive and lighting loads; the load type decides the letters after the number.
Breaker selection with the full margin and derating logic is handled step by step by the Breaker Size Calculator.
Panels, Disconnects, and Boxes
Panel sizing
A panel’s main breaker rating (100, 150, 200, 400 A) must cover the home’s total calculated load — not the sum of all branch breakers, which always exceeds it. Build that total with the estimating guide method, then structure it with the Electrical Load Calculator. Also count spaces: a panel with no spare breaker positions forces a subpanel or panel replacement even when load capacity remains.
Disconnects
Large hardwired loads (A/C condensers, hot tubs, workshops) need a local disconnect within sight of the equipment, rated at or above the load current. Size it like a breaker: next standard size up from the load, ≥ 115–125% for motors per NEC 430.
Boxes and conduit
Box fill rules (NEC 314.16) limit how many conductors a box can hold — larger wire eats box volume fast (each 12 AWG conductor counts 2.25 in³). Conduit fill (Chapter 9 tables) similarly limits conductors per raceway, which interacts with the derating rules above. When upsizing wire for a long run, verify the existing conduit and boxes still comply.
Connectors and Terminations
Often overlooked, always critical: every splice and termination must be rated for the wire size, the current, and (for aluminum) the metal. Wire nuts are sized by wire combination — the package chart, not habit, decides. Torque matters on breaker and panel lugs: manufacturer torque specs exist because under-torqued lugs overheat. A $30 torque screwdriver is cheap insurance on any panel work.
Related Guides
- Code Compliance — the NEC articles behind every selection rule here.
- Estimating Guide — total the load before selecting the panel.
- Cost Guide — what this hardware costs at each amperage.
- Common Mistakes — selection errors and how to avoid them.
- Complete Guide — the full amperage reference.
Frequently Asked Questions
Can I use 14 AWG wire on a 20 amp breaker?
No — 14 AWG copper is rated 15 A, and the breaker must not exceed the wire’s ampacity. Use 12 AWG for 20 A circuits.
Is aluminum wire safe?
Modern AA-8000 series aluminum is safe for feeders and service entrances when terminated with aluminum-rated lugs and proper technique. The 1970s-era branch-circuit aluminum problems came from old alloys and improper terminations — a different situation from today’s feeder applications.
When does voltage drop force larger wire?
Typically past ~100 feet (30 m) on 120 V branch circuits. If calculated drop exceeds ~3%, upsize the wire — the Voltage Drop Calculator gives the exact answer for your run.
Do I need GFCI, AFCI, or both?
It depends on location and circuit type under your locally adopted NEC edition — wet locations generally need GFCI, most living-area 15/20 A circuits need AFCI, and some circuits need both (dual-function breakers exist). Your AHJ’s adopted code edition is the final authority.
Get Your Number
Material selection is a chain: load current → 80% rule → derating → wire ampacity → breaker protecting that wire → panel with spare capacity. The Amperage Calculator starts the chain with the current; this guide carries it through the hardware. Size each link for the derated reality, not the catalog best case.
Disclaimer: planning estimates only — ampacity values are baselines subject to NEC derating and local amendments. Material selection for real installations must be verified with a licensed electrician and your local authority having jurisdiction.
